A robot wrist structure that includes a first wrist element rotatable about a first axis, a second wrist element supported by the first wrist element so as to be rotatable about a second axis orthogonal to the first axis, and a third wrist element supported by the second wrist element so as to be rotatable about a third axis orthogonal to the second axis is known in the art (for example, refer to Patent Literature 1).
In this wrist structure, powers output from a second wrist motor and a third wrist motor installed in the first wrist element are transmitted to the second wrist element and the third wrist element via two hypoid gear sets each equipped with a pinion gear and a ring gear.
The two ring gears are coaxially arranged. The ring gear on the radially outer side drives the second wrist element by reducing the speed of rotation of one of the wrist motors, and the ring gear on the radially inner side drives the third wrist element via a bevel gear inside a second driving element by reducing the speed of rotation of the other wrist motor.
According to the wrist structure described in Patent Literature 1, the second wrist element includes a first housing equipped with an output flange fixed to the ring gear on the radially outer side, a second housing fixed to the output flange of the first housing, and a third housing fixed to the first housing and the second housing. The first housing houses a drive-side bevel gear of the third wrist element fixed to the ring gear on the radially inner side. The third housing rotatably supports the third wrist element and houses a driven-side bevel gear.
The second housing functions as a component that connects the first housing to the third housing. These three housings are fastened to one another with screws and fixed by being set in place with locating pins.
Patent Literature 1, described above, is the Publication of Japanese Patent No. 4233578.